1. Field of the Invention
The present invention relates to a fusing unit for an image forming apparatus such as a copying machine, a facsimile machine, or a printer, for example, and more particularly to an improvement of a structure of a separating pawl that forcedly separates a sheet from a circumferential surface of a heat roller.
2. Description of the Related Art
In a fusing unit of an image forming apparatus of the related art, the separating pawls are disposed at six points in a circumferential surface of a heat roller. The six separating pawls are constructed by three groups of pairs of separating pawls. In the first group, the separating pawls are disposed at two points on the right and left in the center of the heat roller. In the second group, the separating pawls are disposed outside the separating pawls of the first group. In the third group, the separating pawls are disposed on both side ends of the heat roller. The separating pawls of the third group are pressed against the circumferential surface of the heat roller by a spring having an elastic force larger than that of the separating pawls of the first and second groups. An image forming area of the sheet is separated from the heat roller by the separating pawls of the first and second groups, which are biased by a force that is smaller than that of the separating pawls of the third group, so that the fused image can be prevented from being damaged by the separating pawls.
In another fusing unit of the related art, the separating pawls are disposed at five points along the circumferential surface of the heat roller. The five separating pawls are constructed by central separating pawls that are disposed at three points in the center of the heat roller and two side separating pawls that are disposed on both side ends of the heat roller. Although the central separating pawls are substantially identical to the separating pawls in a basic configuration, the central separating pawls differs from the separating pawls in a structure of a rear end portion of a guide surface that is continuously provided adjacent to a tip end of the separating pawl. The rear end portion of the guide surface of the central separating pawl is drawn from the rear end portion of the guide surface of the side separating pawl with respect to the sheet. Therefore, the central separating pawl is prevented from providing a large frictional force, and generation of a streak flaw can be prevented in the central portion in a width direction of the sheet.
In still another fusing unit of the related art, the sheet is guided toward a sheet discharge guide by a rotatable guide provided in the separating pawl. The rotatable guide is constructed by a sprocket.
Generally, five or six separating pawls are provided in the fusing unit of the image forming apparatus of the related art, and the post-fusing sheet can stably be fed toward the sheet discharge guide near an exit roller because the sheet can be guided by more separating pawls as a sheet size is enlarged. There is no problem in a finishing state of the post-fusing sheet. However, in the case of a small-size sheet such as a postcard, because the sheet is guided by one or two separating pawls provided in the center in the circumferential surface of the heat roller, a large pressure is applied to the sheet, and sometimes a problem is generated in the finishing of the post-fusing sheet.
Particularly, in the case that a feed path is largely flexed between a nip portion of the fusing unit and the sheet discharge guide, a portion of a length in a feed direction of the small-size sheet such as the postcard is flexed, and thus, a sheet plane is not free from application of a large amount of pressure. In the case that the sheet is guided while an orientation of the sheet is changed by the sprocket provided in the separating pawl, because the pressure applied to the sheet plane is concentrated, a broken-line pattern matched with a pitch of the teeth of the sprocket is formed in a rear surface of the sheet, thereby degrading the finishing state of the sheet.
For example, when the central separating pawl that separates the small-size sheet is eliminated, the pressure applied to the sheet plane can be reduced to relax a sheet feeding condition, and the same finishing state as the large-size sheet is obtained. However, in fusing a thin and weak sheet, it is difficult to stably feed the sheet, and there is generated a new problem in that a corner portion at a start end edge in the feed direction of the sheet is folded or a wrinkle.